U.S. patent application number 13/920492 was filed with the patent office on 2013-10-31 for elastic wave device and method for manufacturing the same.
The applicant listed for this patent is Murata Manufacturing Co., Ltd.. Invention is credited to Hideki IWAMOTO, Hajime KANDO, Syunsuke KIDO, Munehisa WATANABE.
Application Number | 20130285768 13/920492 |
Document ID | / |
Family ID | 46313908 |
Filed Date | 2013-10-31 |
United States Patent
Application |
20130285768 |
Kind Code |
A1 |
WATANABE; Munehisa ; et
al. |
October 31, 2013 |
ELASTIC WAVE DEVICE AND METHOD FOR MANUFACTURING THE SAME
Abstract
An elastic wave device includes a supporting substrate, a
high-acoustic-velocity film stacked on the supporting substrate and
in which an acoustic velocity of a bulk wave propagating therein is
higher than an acoustic velocity of an elastic wave propagating in
a piezoelectric film, a low-acoustic-velocity film stacked on the
high-acoustic-velocity film and in which an acoustic velocity of a
bulk wave propagating therein is lower than an acoustic velocity of
a bulk wave propagating in the piezoelectric film, the
piezoelectric film is stacked on the low-acoustic-velocity film,
and an IDT electrode stacked on a surface of the piezoelectric
film.
Inventors: |
WATANABE; Munehisa;
(Nagaokakyo-shi, JP) ; IWAMOTO; Hideki;
(Nagaokakyo-shi, JP) ; KANDO; Hajime;
(Nagaokakyo-shi, JP) ; KIDO; Syunsuke;
(Nagaokakyo-shi, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Murata Manufacturing Co., Ltd. |
Nagaokakyo-shi |
|
JP |
|
|
Family ID: |
46313908 |
Appl. No.: |
13/920492 |
Filed: |
June 18, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
PCT/JP2011/079496 |
Dec 20, 2011 |
|
|
|
13920492 |
|
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|
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Current U.S.
Class: |
333/193 |
Current CPC
Class: |
H03H 3/02 20130101; H01L
41/0477 20130101; H03H 9/0222 20130101; Y10T 29/49155 20150115;
H03H 9/54 20130101; H03H 2003/027 20130101; H03H 9/02834 20130101;
H03H 9/02574 20130101; H01L 41/18 20130101; H01L 41/22 20130101;
H03H 3/04 20130101; H03H 3/08 20130101; Y10T 29/42 20150115; H03H
3/10 20130101; Y10T 29/49005 20150115; H01L 41/04 20130101; H01L
41/047 20130101; H03H 2003/023 20130101 |
Class at
Publication: |
333/193 |
International
Class: |
H03H 9/54 20060101
H03H009/54 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 24, 2010 |
JP |
2010-288453 |
Claims
1. An elastic wave device including a piezoelectric film, the
elastic wave device comprising: a high-acoustic-velocity supporting
substrate in which an acoustic velocity of a bulk wave propagating
therein is higher than an acoustic velocity of an elastic wave
propagating in the piezoelectric film; a low-acoustic-velocity film
stacked on the high-acoustic-velocity supporting substrate, in
which an acoustic velocity of a bulk wave propagating therein is
lower than an acoustic velocity of a bulk wave propagating in the
piezoelectric film; the piezoelectric film stacked on the
low-acoustic-velocity film; and an IDT electrode disposed on a
surface of the piezoelectric film.
2. The elastic wave device according to claim 1, wherein a portion
of energy of an elastic wave propagating in the piezoelectric film
is distributed into the low-acoustic-velocity film and the
high-acoustic-velocity supporting substrate.
3. An elastic wave device including a piezoelectric film, the
elastic wave device comprising: a supporting substrate; a
high-acoustic-velocity film disposed on the supporting substrate,
in which an acoustic velocity of a bulk wave propagating therein is
higher than an acoustic velocity of an elastic wave propagating in
the piezoelectric film; a low-acoustic-velocity film stacked on the
high-acoustic-velocity film, in which an acoustic velocity of a
bulk wave propagating therein is lower than an acoustic velocity of
a bulk wave propagating in the piezoelectric film; the
piezoelectric film stacked on the low-acoustic-velocity film; and
an IDT electrode disposed on a surface of the piezoelectric
film.
4. The elastic wave device according to claim 3, wherein a portion
of energy of an elastic wave propagating in the piezoelectric film
is distributed into the low-acoustic-velocity film and the
high-acoustic-velocity film.
5. The elastic wave device according to claim 1, wherein the
low-acoustic-velocity film is composed of silicon oxide or a film
containing as a major component silicon oxide.
6. The elastic wave device according to claim 1, wherein a
thickness of the piezoelectric film is about 1.5.lamda. or less,
where .lamda. is a wavelength of an elastic wave determined by an
electrode period of the IDT electrode.
7. The elastic wave device according to of claim 1, wherein a
thickness of the piezoelectric film is in a range of about
0.05.lamda. to about 0.5.lamda., where .lamda. is a wavelength of
an elastic wave determined by an electrode period of the IDT
electrode.
8. The elastic wave device according to claim 1, wherein a
thickness of the low-acoustic-velocity film is about 2.lamda. or
less, where .lamda. is a wavelength of an elastic wave determined
by an electrode period of the IDT electrode.
9. The elastic wave device according to claim 1, wherein the
piezoelectric film is composed of single-crystal lithium tantalate
with Euler angles (0.+-.5.degree., .theta., .psi.), and the Euler
angles (0.+-.5.degree., .theta., .psi.) are located in any one of a
plurality of regions R1 shown in FIG. 17.
10. The elastic wave device according to claim 9, wherein the Euler
angles (0.+-.5.degree., .theta., .psi.) of the piezoelectric film
are located in any one of a plurality of regions R2 shown in FIG.
18.
11. The elastic wave device according to claim 1, wherein a
coefficient of linear expansion of the supporting substrate is
lower than that of the piezoelectric film.
12. The elastic wave device according to claim 1, wherein a
specific acoustic impedance of the low-acoustic-velocity film is
lower than that of the piezoelectric film.
13. The elastic wave device according to claim 1, wherein a
dielectric film is disposed on the piezoelectric film and the IDT
electrode, and a surface acoustic wave propagates in the
piezoelectric film.
14. The elastic wave device according to claim 1, wherein a
dielectric film is disposed on the piezoelectric film and the IDT
electrode, and a boundary acoustic wave propagates along a boundary
between the piezoelectric film and the dielectric film.
15. The elastic wave device according to claim 1, wherein at least
one of an adhesion layer, an underlying film, a
low-acoustic-velocity layer, and a high-acoustic-velocity layer is
disposed in at least one of boundaries between the piezoelectric
film, the low-acoustic-velocity film, the high-acoustic-velocity
film, and the supporting substrate.
16. A method for manufacturing an elastic wave device comprising: a
step of preparing a supporting substrate; a step of forming a
high-acoustic-velocity film, in which an acoustic velocity of a
bulk wave propagating therein is higher than an acoustic velocity
of an elastic wave propagating in a piezoelectric, on the
supporting substrate; a step of forming a low-acoustic-velocity
film, in which an acoustic velocity of a bulk wave propagating
therein is lower than an acoustic velocity of a bulk wave
propagating in a piezoelectric, on the high-acoustic-velocity film;
a step of forming a piezoelectric layer on the
low-acoustic-velocity film; and a step of forming an IDT electrode
on a surface of the piezoelectric layer.
17. The method for manufacturing an elastic wave device according
to claim 16, wherein the steps of forming the
high-acoustic-velocity film, the low-acoustic-velocity film, and
the piezoelectric layer on the supporting substrate include: (a) a
step of performing ion implantation from a surface of a
piezoelectric substrate having a larger thickness than that of the
piezoelectric layer; (b) a step of forming the
low-acoustic-velocity film on the surface of the piezoelectric
substrate on which the ion implantation has been performed; (c) a
step of forming the high-acoustic-velocity film on a surface of the
low-acoustic-velocity film, opposite to the piezoelectric substrate
side of the low-acoustic-velocity film; (d) a step of bonding the
supporting substrate to a surface of the high-acoustic-velocity
film, opposite to the surface on which the low-acoustic-velocity
film is stacked; and (e) a step of, while heating the piezoelectric
substrate, separating a piezoelectric film, at a high concentration
ion-implanted region of the piezoelectric substrate in which the
implanted ion concentration is highest, from a remaining portion of
the piezoelectric substrate such that the piezoelectric film
remains on a low-acoustic-velocity film side.
18. The method for manufacturing an elastic wave device according
to claim 17, further comprising a step of, after separating the
remaining portion of the piezoelectric substrate, heating the
piezoelectric film disposed on the low-acoustic-velocity film to
recover piezoelectricity.
19. The method for manufacturing an elastic wave device according
to claim 16, further comprising a step of, prior to bonding the
supporting substrate, performing mirror finishing on the surface of
the high-acoustic-velocity film, opposite to the
low-acoustic-velocity film side of the high-acoustic-velocity film.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to an elastic wave device
preferably for use in a resonator, a bandpass filter, or the like
and a method for manufacturing the same. More particularly, the
present invention relates to an elastic wave device having a
structure including a supporting substrate, a piezoelectric layer,
and a layer of another material disposed therebetween, and a method
for manufacturing the same.
[0003] 2. Description of the Related Art
[0004] Elastic wave devices have been widely used as resonators and
bandpass filters, and in recent years, there has been a need for
increasing the frequency thereof. Japanese Unexamined Patent
Application Publication No. 2004-282232 described below discloses a
surface acoustic wave device in which a hard dielectric layer, a
piezoelectric film, and an IDT electrode are stacked in that order
on a dielectric substrate. In such a surface acoustic wave device,
by disposing the hard dielectric layer between the dielectric
substrate and the piezoelectric film, an increase in the acoustic
velocity of surface acoustic waves is achieved. It is described
that thereby, the frequency of the surface acoustic wave device can
be increased.
[0005] Japanese Unexamined Patent Application Publication No.
2004-282232 also discloses a structure in which a potential
equalizing layer is provided between the hard dielectric layer and
the piezoelectric film. The potential equalizing layer is composed
of a metal or semiconductor. The potential equalizing layer is
provided in order to equalize the potential at the interface
between the piezoelectric film and the hard dielectric layer.
[0006] In the surface acoustic wave device described in Japanese
Unexamined Patent Application Publication No. 2004-282232, an
increase in acoustic velocity is achieved by forming the hard
dielectric layer. However, there is considerable propagation loss,
and surface acoustic waves cannot be effectively confined within
the piezoelectric thin film. Consequently, the energy of the
surface acoustic wave device leaks into the dielectric substrate,
and therefore, it is not possible to enhance the Q factor, which is
a problem.
SUMMARY OF THE INVENTION
[0007] Preferred embodiments of the present invention provide an
elastic wave device having a high Q factor and a method for
manufacturing the same.
[0008] An elastic wave device including a piezoelectric film
according to a preferred embodiment of the present invention
includes a high-acoustic-velocity supporting substrate in which the
acoustic velocity of a bulk wave propagating therein is higher than
the acoustic velocity of an elastic wave propagating in the
piezoelectric film; a low-acoustic-velocity film stacked on the
high-acoustic-velocity supporting substrate, in which the acoustic
velocity of a bulk wave propagating therein is lower than the
acoustic velocity of a bulk wave propagating in the piezoelectric
film; the piezoelectric film stacked on the low-acoustic-velocity
film; and an IDT electrode disposed on a surface of the
piezoelectric film.
[0009] The elastic wave device including a piezoelectric film
according to a preferred embodiment of the present invention is
structured such that some portion of energy of an elastic wave
propagating in the piezoelectric film is distributed into the
low-acoustic-velocity film and the high-acoustic-velocity
supporting substrate.
[0010] An elastic wave device including a piezoelectric film
according to a preferred embodiment of the present invention
includes a supporting substrate; a high-acoustic-velocity film
disposed on the supporting substrate, in which the acoustic
velocity of a bulk wave propagating therein is higher than the
acoustic velocity of an elastic wave propagating in the
piezoelectric film; a low-acoustic-velocity film stacked on the
high-acoustic-velocity film, in which the acoustic velocity of a
bulk wave propagating therein is lower than the acoustic velocity
of a bulk wave propagating in the piezoelectric film; the
piezoelectric film stacked on the low-acoustic-velocity film; and
an IDT electrode disposed on a surface of the piezoelectric
film.
[0011] The elastic wave device including a piezoelectric film
according to a preferred embodiment of the present invention is
structured such that some portion of energy of an elastic wave
propagating in the piezoelectric film is distributed into the
low-acoustic-velocity film and the high-acoustic-velocity film.
[0012] In a specific aspect of the elastic wave device according to
a preferred embodiment of the present invention, the
low-acoustic-velocity film is preferably made of silicon oxide or a
film containing as a major component silicon oxide. In such a case,
the absolute value of the temperature coefficient of frequency TCF
can be decreased. Furthermore, the electromechanical coupling
coefficient can be increased, and the band width ratio can be
enhanced. That is, an improvement in temperature characteristics
and an enhancement in the band width ratio can be simultaneously
achieved.
[0013] In another specific aspect of the elastic wave device
according to a preferred embodiment of the present invention, the
thickness of the piezoelectric film preferably is about 1.5.lamda.
or less, for example, where .lamda. is the wavelength of an elastic
wave determined by the electrode period of the IDT electrode. In
such a case, by selecting the thickness of the piezoelectric film
in a range of about 1.5.lamda. or less, for example, the
electromechanical coupling coefficient can be easily adjusted.
[0014] In another specific aspect of the elastic wave device
according to a preferred embodiment of the present invention, by
selecting the thickness of the piezoelectric film in a range of
about 0.05.lamda. to about 0.5.lamda., where .lamda. is the
wavelength of an elastic wave determined by the electrode period of
the IDT electrode, the electromechanical coupling coefficient can
be easily adjusted over a wide range.
[0015] In another specific aspect of the elastic wave device
according to a preferred embodiment of the present invention, the
thickness of the low-acoustic-velocity film preferably is a
preferred embodiment of 2.lamda. or less, for example, where
.lamda. is the wavelength of an elastic wave determined by the
electrode period of the IDT electrode. In such a case, by selecting
the thickness of the low-acoustic-velocity film in a range of about
2.lamda. or less, for example, the electromechanical coupling
coefficient can be easily adjusted. Furthermore, the warpage of the
elastic wave device due to the film stress of the
low-acoustic-velocity film can be reduced. Consequently, freedom of
design can be increased, and it is possible to provide a
high-quality elastic wave device which is easy to handle.
[0016] In another specific aspect of the elastic wave device
according to a preferred embodiment of the present invention, the
piezoelectric film is preferably made of single-crystal lithium
tantalate with Euler angles (0.+-.5.degree., .theta., .psi.), and
the Euler angles (0.+-.5.degree., .theta., .psi.) are located in
any one of a plurality of regions R1 shown in FIG. 17. In such a
case, the electromechanical coupling coefficient of the SH
component of the elastic wave can be set at about 2% or more, for
example. Consequently, the electromechanical coupling coefficient
can be sufficiently increased.
[0017] In another specific aspect of the elastic wave device
according to a preferred embodiment of the present invention, the
piezoelectric film is preferably made of single-crystal lithium
tantalate with Euler angles (0.+-.5.degree., .theta., .psi.), and
the Euler angles (0.+-.5.degree., .theta., .psi.) are located in
any one of a plurality of regions R2 shown in FIG. 18. In such a
case, the electromechanical coupling coefficient of the SH
component used can be increased, and the SV wave, which is
spurious, can be effectively suppressed.
[0018] In another specific aspect of the elastic wave device
according to a preferred embodiment of the present invention, the
coefficient of linear expansion of the supporting substrate is
lower than that of the piezoelectric film. In such a case, the
temperature characteristics can be further improved.
[0019] In another specific aspect of the elastic wave device
according to a preferred embodiment of the present invention, the
specific acoustic impedance of the low-acoustic-velocity film is
lower than that of the piezoelectric film. In such a case, the band
width ratio can be further enhanced.
[0020] In another specific aspect of the elastic wave device
according to a preferred embodiment of the present invention, a
dielectric film is disposed on the piezoelectric film and the IDT
electrode, and a surface acoustic wave propagates in the
piezoelectric film.
[0021] In another specific aspect of the elastic wave device
according to a preferred embodiment of the present invention, a
dielectric film is disposed on the piezoelectric film and the IDT
electrode, and a boundary acoustic wave propagates along a boundary
between the piezoelectric film and the dielectric film.
[0022] In another specific aspect of the elastic wave device
according to a preferred embodiment of the present invention, at
least one of an adhesion layer, an underlying film, a
low-acoustic-velocity layer, and a high-acoustic-velocity layer is
disposed in at least one of boundaries between the piezoelectric
film, the low-acoustic-velocity film, the high-acoustic-velocity
film, and the supporting substrate.
[0023] A method for manufacturing an elastic wave device according
to yet another preferred embodiment of the present invention
includes a step of preparing a supporting substrate; a step of
forming a high-acoustic-velocity film, in which the acoustic
velocity of a bulk wave propagating therein is higher than the
acoustic velocity of an elastic wave propagating in a
piezoelectric, on the supporting substrate; a step of forming a
low-acoustic-velocity film, in which the acoustic velocity of a
bulk wave propagating therein is lower than the acoustic velocity
of a bulk wave propagating in a piezoelectric, on the
high-acoustic-velocity film; a step of forming a piezoelectric
layer on the low-acoustic-velocity film; and a step of forming an
IDT electrode on a surface of the piezoelectric layer.
[0024] In a specific aspect of the method for manufacturing an
elastic wave device according to a preferred embodiment of the
present invention, the steps of forming the high-acoustic-velocity
film, the low-acoustic-velocity film, and the piezoelectric layer
on the supporting substrate include the following steps (a) to
(e).
[0025] (a) A step of performing ion implantation from a surface of
a piezoelectric substrate having a larger thickness than that of
the piezoelectric layer.
[0026] (b) A step of forming the low-acoustic-velocity film on the
surface of the piezoelectric substrate on which the ion
implantation has been performed.
[0027] (c) A step of forming the high-acoustic-velocity film on a
surface of the low-acoustic-velocity film, opposite to the
piezoelectric substrate side of the low-acoustic-velocity film.
[0028] (d) A step of bonding the supporting substrate to a surface
of the high-acoustic-velocity film, opposite to the surface on
which the low-acoustic-velocity film is stacked.
[0029] (e) A step of, while heating the piezoelectric substrate,
separating a piezoelectric film, at a high concentration
ion-implanted region of the piezoelectric substrate in which the
implanted ion concentration is highest, from a remaining portion of
the piezoelectric substrate such that the piezoelectric film
remains on the low-acoustic-velocity film side.
[0030] In another specific aspect of the method for manufacturing
an elastic wave device according to a preferred embodiment of the
present invention, the method further includes a step of, after
separating the remaining portion of the piezoelectric substrate,
heating the piezoelectric film disposed on the
low-acoustic-velocity film to recover piezoelectricity. In such a
case, since the piezoelectricity of the piezoelectric film can be
recovered by heating, it is possible to provide an elastic wave
device having good characteristics.
[0031] In another specific aspect of the method for manufacturing
an elastic wave device according to a preferred embodiment of the
present invention, the method further includes a step of, prior to
bonding the supporting substrate, performing mirror finishing on
the surface of the high-acoustic-velocity film, opposite to the
low-acoustic-velocity film side of the high-acoustic-velocity film.
In such a case, it is possible to strengthen bonding between the
high-acoustic-velocity film and the supporting substrate.
[0032] In the elastic wave device according to various preferred
embodiments of the present invention, since the
high-acoustic-velocity film and the low-acoustic-velocity film are
disposed between the supporting substrate and the piezoelectric
film, the Q factor can be enhanced. Consequently, it is possible to
provide an elastic wave device having a high Q factor.
[0033] Furthermore, in the manufacturing method according to
various preferred embodiments of the present invention, it is
possible to provide an elastic wave device of the present invention
having a high Q factor.
[0034] The above and other elements, features, steps,
characteristics and advantages of the present invention will become
more apparent from the following detailed description of the
preferred embodiments with reference to the attached drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0035] FIG. 1A is a schematic elevational cross-sectional view of a
surface acoustic wave device according to a first preferred
embodiment of the present invention, and FIG. 1B is a schematic
plan view showing an electrode structure of the surface acoustic
wave device.
[0036] FIG. 2 is a graph showing impedance characteristics of
surface acoustic wave devices of the first preferred embodiment, a
first comparative example, and a second comparative example.
[0037] FIG. 3 shows an impedance Smith chart for the surface
acoustic wave devices of the first preferred embodiment, the first
comparative example, and the second comparative example.
[0038] FIGS. 4A and 4B are each a graph showing the results of
simulation in the IDT electrode portion of the surface acoustic
wave device of the first preferred embodiment of the present
invention, regarding the relationship between the AlN film
thickness and the percentage of energy concentration.
[0039] FIG. 5 is a graph showing the results of FEM simulation of
impedance characteristics of surface acoustic wave devices
according to the first preferred embodiment and the first and
second comparative examples.
[0040] FIG. 6 is a graph showing the results of FEM simulation of
the relationship between the Q factor and the frequency in the
surface acoustic wave devices of the first preferred embodiment and
the first and second comparative examples.
[0041] FIG. 7 is a graph showing the relationship between the
thickness of the piezoelectric film composed of LiTaO.sub.2, the
acoustic velocity, and the normalized film thickness of the
low-acoustic-velocity film composed of SiO.sub.2 in a surface
acoustic wave device according to a preferred embodiment of the
present invention.
[0042] FIG. 8 is a graph showing the relationship between the
thickness of the piezoelectric film composed of LiTaO.sub.3, the
normalized film thickness of the low-acoustic-velocity film
composed of SiO.sub.2, and the electromechanical coupling
coefficient in the surface acoustic wave device according to a
preferred embodiment of the present invention.
[0043] FIG. 9 is a graph showing the relationship between the
thickness of the piezoelectric film composed of LiTaO.sub.3, the
normalized film thickness of the low-acoustic-velocity film
composed of SiO.sub.2, and the TCV in a surface acoustic wave
device according to a preferred embodiment of the present
invention.
[0044] FIG. 10 is a graph showing the relationship between the
thickness of the piezoelectric film composed of LiTaO.sub.3, the
normalized film thickness of the low-acoustic-velocity film
composed of SiO.sub.2, and the band width ratio in a surface
acoustic wave device according to a preferred embodiment of the
present invention.
[0045] FIG. 11 is a graph showing the relationship between the band
width ratio BW and the TCF in surface acoustic wave devices of
third to fifth comparative examples.
[0046] FIG. 12 is a graph showing the relationship between the band
width ratio, the temperature coefficient of frequency TCV, and the
normalized film thickness of the low-acoustic-velocity film in the
surface acoustic wave device of the first preferred embodiment of
the present invention.
[0047] FIG. 13 is a graph showing the relationship between the
thickness of the piezoelectric film composed of LiTaO.sub.3 and the
acoustic velocity at the resonance point as well as the acoustic
velocity at the antiresonance point in the surface acoustic wave
device according to a second preferred embodiment of the present
invention.
[0048] FIG. 14 is a graph showing the relationship between the
thickness of the piezoelectric film composed of LiTaO.sub.3 and the
band width ratio in the surface acoustic wave device according to
the second preferred embodiment of the present invention.
[0049] FIG. 15 is a graph showing the relationship between the
normalized film thickness of the SiO.sub.2 film and the material
for the high-acoustic-velocity film in surface acoustic wave
devices according to a third preferred embodiment of the present
invention.
[0050] FIG. 16 is a graph showing the relationship between the
normalized film thickness of the SiO.sub.2 film, the
electromechanical coupling coefficient, and the material for the
high-acoustic-velocity film in surface acoustic wave devices
according to the third preferred embodiment of the present
invention.
[0051] FIG. 17 is a graph showing a plurality of regions R1 in
which the electromechanical coupling coefficient of the surface
acoustic wave mode containing as a major component the U2 (SH) mode
preferably is about 2% or more in the LiTaO.sub.3 films with Euler
angles (0.+-.5.degree., .theta., .psi.) of surface acoustic wave
devices according to a fourth preferred embodiment of the present
invention.
[0052] FIG. 18 is a graph showing a plurality of regions R2 in
which the electromechanical coupling coefficient of the surface
acoustic wave mode containing as a major component the U2 mode
preferably is about 2% or more and the electromechanical coupling
coefficient of the surface acoustic wave mode containing as a major
component the U3 (SV) mode, which is spurious, preferably is about
1% or less in the LiTaO.sub.3 films with Euler angles
(0.+-.5.degree., .theta., .psi.) of surface acoustic wave devices
according to the fifth preferred embodiment of the present
invention.
[0053] FIGS. 19A to 19C are graphs showing the relationships
between the specific acoustic impedance of the
low-acoustic-velocity film and the band width ratio in surface
acoustic wave devices according to a sixth preferred embodiment of
the present invention.
[0054] FIGS. 20A to 20C are graphs showing the relationships
between the specific acoustic impedance of the
low-acoustic-velocity film and the acoustic velocity of the surface
acoustic wave in surface acoustic wave devices according to the
sixth preferred embodiment of the present invention.
[0055] FIGS. 21A to 21E are elevational cross-sectional views for
explaining a method for manufacturing a surface acoustic wave
device according to the first preferred embodiment of the present
invention.
[0056] FIGS. 22A to 22C are elevational cross-sectional views for
explaining the method for manufacturing a surface acoustic wave
device according to the first preferred embodiment of the present
invention.
[0057] FIG. 23 is a schematic elevational cross-sectional view of a
surface acoustic wave device according to a seventh preferred
embodiment of the present invention.
[0058] FIG. 24 is a schematic elevational cross-sectional view of a
surface acoustic wave device according to an eighth preferred
embodiment of the present invention.
[0059] FIG. 25 is a schematic elevational cross-sectional view of a
surface acoustic wave device according to a ninth preferred
embodiment of the present invention.
[0060] FIG. 26 is a graph showing the relationship between the
SiO.sub.2 film thickness and the Q.sub.max factor in the case where
the thickness of piezoelectric thin films of surface acoustic wave
devices according to a tenth preferred embodiment of the present
invention is changed.
[0061] FIG. 27 is a schematic elevational cross-sectional view of a
boundary acoustic wave device according to an eleventh preferred
embodiment of the present invention.
[0062] FIG. 28 is a schematic elevational cross-sectional view of a
boundary acoustic wave device according to a twelfth preferred
embodiment of the present invention.
[0063] FIG. 29 is a schematic elevational cross-sectional view of a
boundary acoustic wave device according to a thirteenth preferred
embodiment of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0064] The present invention will be clarified by describing
specific preferred embodiments of the present invention with
reference to the drawings.
[0065] FIG. 1A is a schematic elevational cross-sectional view of a
surface acoustic wave device according to a first preferred
embodiment of the present invention.
[0066] A surface acoustic wave device 1 includes a supporting
substrate 2. A high-acoustic-velocity film 3 having a relatively
high acoustic velocity is stacked on the supporting substrate 2. A
low-acoustic-velocity film 4 having a relatively low acoustic
velocity is stacked on the high-acoustic-velocity film 3. A
piezoelectric film 5 is stacked on the low-acoustic-velocity film
4. An IDT electrode 6 is stacked on the upper surface of the
piezoelectric film 5. Note that the IDT electrode 6 may be disposed
on the lower surface of the piezoelectric film 5.
[0067] The supporting substrate 2 may be composed of an appropriate
material as long as it can support the laminated structure
including the high-acoustic-velocity film 3, the
low-acoustic-velocity film 4, the piezoelectric film 5, and the IDT
electrode 6. Examples of such a material that can be used include
piezoelectrics, such as sapphire, lithium tantalate, lithium
niobate, and quartz; various ceramics, such as alumina, magnesia,
silicon nitride, aluminum nitride, silicon carbide, zirconia,
cordierite, mullite, steatite, and forsterite; dielectrics, such as
glass; semiconductors, such as silicon and gallium nitride; and
resin substrates. In this preferred embodiment, the supporting
substrate 2 is preferably composed of glass.
[0068] The high-acoustic-velocity film 3 functions in such a manner
that a surface acoustic wave is confined to a portion in which the
piezoelectric film 5 and the low-acoustic-velocity film 4 are
stacked and the surface acoustic wave does not leak into the
structure below the high-acoustic-velocity film 3. In this
preferred embodiment, the high-acoustic-velocity film 3 is
preferably composed of aluminum nitride. As the material for
high-acoustic-velocity film 3, as long as it is capable of
confining the elastic wave, any of various high-acoustic-velocity
materials can be used. Examples thereof include aluminum nitride,
aluminum oxide, silicon carbide, silicon nitride, silicon
oxynitride, a DLC film or diamond, media mainly composed of these
materials, and media mainly composed of mixtures of these
materials. In order to confine the surface acoustic wave to the
portion in which the piezoelectric film 5 and the
low-acoustic-velocity film 4 are stacked, it is preferable that the
thickness of the high-acoustic-velocity film 3 be as large as
possible. The thickness of the high-acoustic-velocity film 3 is
preferably about 0.5 times or more, more preferably about 1.5 times
or more, than the wavelength .lamda. of the surface acoustic
wave.
[0069] In this description, the "high-acoustic-velocity film" is
defined as a film in which the acoustic velocity of a bulk wave
propagating therein is higher than the acoustic velocity of an
elastic wave, such as a surface acoustic wave or a boundary
acoustic wave, propagating in or along the piezoelectric film 5.
Furthermore, the "low-acoustic-velocity film" is defined as a film
in which the acoustic velocity of a bulk wave propagating therein
is lower than the acoustic velocity of a bulk wave propagating in
the piezoelectric film 5. Furthermore, elastic waves with various
modes having different acoustic velocities are excited by an IDT
electrode having a certain structure. The "elastic wave propagating
in the piezoelectric film 5" represents an elastic wave with a
specific mode used for obtaining filter or resonator
characteristics. The bulk wave mode that determines the acoustic
velocity of the bulk wave is defined in accordance with the usage
mode of the elastic wave propagating in the piezoelectric film 5.
In the case where the high-acoustic-velocity film 3 and the
low-acoustic-velocity film 4 are isotropic with respect to the
propagation direction of the bulk wave, correspondences are as
shown in Table 1 below. That is, for the dominant mode of the
elastic wave shown in the left column of Table 1, the high acoustic
velocity and the low acoustic velocity are determined according to
the mode of the bulk wave shown in the right column of Table 1. The
P wave is a longitudinal wave, and the S wave is a transversal
wave.
[0070] In Table 1, U1 represents an elastic wave containing as a
major component a P wave, U2 represents an elastic wave containing
as a major component an SH wave, and U3 represents an elastic wave
containing as a major component an SV wave.
TABLE-US-00001 TABLE 1 Correspondence of the elastic wave mode of
the piezoelectric film to the bulk wave mode of the dielectric film
(in the case where the dielectric film is composed of an isotropic
material) Dominant mode of the Mode of the bulk wave elastic wave
propagating in propagating in the the piezoelectric film dielectric
film U1 P wave U2 S wave U3 + U1 S wave
[0071] In the case where the low-acoustic-velocity film 4 and the
high-acoustic-velocity film 3 are anisotropic with respect to the
propagation of the bulk wave, bulk wave modes that determine the
high acoustic velocity and the low acoustic velocity are shown in
Table 2 below. In addition, in the bulk wave modes, the slower of
the SH wave and the SV wave is referred to as a slow transversal
wave, and the faster of the two is referred to as a fast
transversal wave. Which of the two is the slow transversal wave
depends on the anisotropy of the material. In LiTaO.sub.3 or
LiNbO.sub.3 cut in the vicinity of rotated Y cut, in the bulk wave
modes, the SV wave is the slow transversal wave, and the SH wave is
the fast transversal wave.
TABLE-US-00002 TABLE 2 Correspondence of the elastic wave mode of
the piezoelectric film to the bulk wave mode of the dielectric film
(in the case where the dielectric film is composed of an
anisotropic material) Dominant mode of the Mode of the bulk wave
elastic wave propagating in propagating in the the piezoelectric
film dielectric film U1 .sup. P wave U2 SH wave U3 + U1 SV wave
[0072] In this preferred embodiment, the low-acoustic-velocity film
4 is preferably composed of silicon oxide, and the thickness
thereof preferably is about 0.35.lamda., where .lamda. is the
wavelength of an elastic wave determined by the electrode period of
the IDT electrode.
[0073] As the material constituting the low-acoustic-velocity film
4, it is possible to use any appropriate material having a bulk
wave acoustic velocity that is slower than the acoustic velocity of
the bulk wave propagating in the piezoelectric film 5. Examples of
such a material that can be used include silicon oxide, glass,
silicon oxynitride, tantalum oxide, and media mainly composed of
these materials, such as compounds obtained by adding fluorine,
carbon, or boron to silicon oxide.
[0074] The low-acoustic-velocity film and the
high-acoustic-velocity film are each composed of an appropriate
dielectric material capable of achieving a high acoustic velocity
or a low acoustic velocity that is determined as described
above.
[0075] In this preferred embodiment, the piezoelectric film 5 is
preferably composed of 38.5.degree. Y cut LiTaO.sub.3, i.e.,
LiTaO.sub.3 with Euler angles of (0.degree., 128.5.degree.,
0.degree.), and the thickness thereof preferably is about
0.25.lamda., where .lamda. is the wavelength of a surface acoustic
wave determined by the electrode period of the IDT electrode 6.
However, the piezoelectric film 5 may be composed of LiTaO.sub.3
with other cut angles, or a piezoelectric single crystal other than
LiTaO.sub.3.
[0076] In this preferred embodiment, the IDT electrode 6 is
preferably composed of Al. However, the IDT electrode 6 may be made
of any appropriate metal material, such as Al, Cu, Pt, Au, Ag, Ti,
Ni, Cr, Mo, W, or an alloy mainly composed of any one of these
metals. Furthermore, the IDT electrode 6 may have a structure in
which a plurality of metal films composed of these metals or alloys
are stacked.
[0077] Although schematically shown in FIG. 1A, an electrode
structure shown in FIG. 1B is disposed on the piezoelectric film 5.
That is, the IDT electrode 6 and reflectors 7 and 8 arranged on
both sides in the surface acoustic wave electrode direction of the
IDT electrode 6 are disposed. A one-port-type surface acoustic wave
resonator is thus constituted. However, the electrode structure
including the IDT electrode in the present invention is not
particularly limited, and a modification is possible such that an
appropriate resonator, a ladder filter in which resonators are
combined, a longitudinally coupled filter, a lattice-type filter,
or a transversal type filter is provided.
[0078] The surface acoustic wave device 1 according to the present
preferred embodiment preferably includes the high-acoustic-velocity
film 3, the low-acoustic-velocity film 4, and the piezoelectric
film 5 stacked on each other. Thereby, the Q factor can be
increased. The reason for this is as follows.
[0079] In the related art, it is known that, by disposing a
high-acoustic-velocity film on the lower surface of a piezoelectric
substrate, some portion of a surface acoustic wave propagates while
distributing energy into the high-acoustic-velocity film, and
therefore, the acoustic velocity of the surface acoustic wave can
be increased.
[0080] In contrast, in various preferred embodiments of the present
invention of the present application, since the
low-acoustic-velocity film 4 is disposed between the
high-acoustic-velocity film 3 and the piezoelectric film 5, the
acoustic velocity of an elastic wave is decreased. Energy of an
elastic wave essentially concentrates on a low-acoustic-velocity
medium. Consequently, it is possible to enhance an effect of
confining elastic wave energy to the piezoelectric film 5 and the
IDT in which the elastic wave is excited. Therefore, in accordance
with this preferred embodiment, the loss can be reduced and the Q
factor can be enhanced compared with the case where the
low-acoustic-velocity film 4 is not provided. Furthermore, the
high-acoustic-velocity film 3 functions such that an elastic wave
is confined to a portion in which the piezoelectric film 5 and the
low-acoustic-velocity film 4 are stacked and the elastic wave does
not leak into the structure below the high-acoustic-velocity film
3. That is, in the structure of a preferred embodiment of the
present invention, energy of an elastic wave of a specific mode
used to obtain filter or resonator characteristics is distributed
into the entirety of the piezoelectric film 5 and the
low-acoustic-velocity film 4 and partially distributed into the
low-acoustic-velocity film side of the high-acoustic-velocity film
3, but is not distributed into the supporting substrate 2. The
mechanism of confining the elastic wave by the
high-acoustic-velocity film is similar to that in the case of a
Love wave-type surface acoustic wave, which is a non-leaky SH wave,
and for example, is described in Kenya Hashimoto; "Introduction to
simulation technologies for surface acoustic wave devices";
Realize; pp. 90-91. The mechanism is different from the confinement
mechanism in which a Bragg reflector including an acoustic
multilayer film is used.
[0081] In addition, in this preferred embodiment, since the
low-acoustic-velocity film 4 is preferably composed of silicon
oxide, temperature characteristics can be improved. The elastic
constant of LiTaO.sub.3 has a negative temperature characteristic,
and silicon oxide has a positive temperature characteristic.
Consequently, in the surface acoustic wave device 1, the absolute
value of TCF can be decreased. In addition, the specific acoustic
impedance of silicon oxide is lower than that of LiTaO.sub.3.
Consequently, an increase in the electromechanical coupling
coefficient, i.e., an enhancement in the band width ratio and an
improvement in frequency temperature characteristics can be
simultaneously achieved.
[0082] Furthermore, by adjusting the thickness of the piezoelectric
film 5 and the thickness of each of the high-acoustic-velocity film
3 and the low-acoustic-velocity film 4, as will be described later,
the electromechanical coupling coefficient can be adjusted in a
wide range. Consequently, freedom of design can be increased.
[0083] Specific experimental examples of the surface acoustic wave
device according to the preferred embodiment described above will
be described below to demonstrate the operation and advantageous
effects of the preferred embodiment.
[0084] A surface acoustic wave device 1 according to the first
preferred embodiment and surface acoustic wave devices according to
first and second comparative examples described below were
fabricated.
[0085] First preferred embodiment: Al electrode (thickness:
0.08.lamda.)/38.5.degree. Y cut LiTaO.sub.3 thin film (thickness:
0.25.lamda.)/silicon oxide film (thickness: 0.35.lamda.)/aluminum
nitride film (1.5.lamda.)/supporting substrate composed of glass
stacked in that order from the top.
[0086] First comparative example: electrode composed of Al
(thickness: 0.08.lamda.)/38.5.degree. Y cut LiTaO.sub.3 substrate
stacked in that order from the top. In the first comparative
example, the electrode composed of Al was formed on the LiTaO.sub.3
substrate with a thickness of 350 .mu.m.
[0087] Second comparative example: Al electrode (thickness:
0.08.lamda.)/38.5.degree. Y cut LiTaO.sub.3 film with a thickness
of 0.5.lamda./aluminum nitride film (thickness:
1.5.lamda.)/supporting substrate composed of glass stacked in that
order from the top.
[0088] In each of the surface acoustic wave devices of the first
preferred embodiment and the first and second comparative examples,
the electrode had a one-port-type surface acoustic wave resonator
structure shown in FIG. 1B. The wavelength .lamda. determined by
the electrode period of the IDT electrode was 2 .mu.m. The dominant
mode of the surface acoustic wave propagating in the 38.5.degree. Y
cut LiTaO.sub.3 is the U2 mode, and its acoustic velocity is about
3,950 m/sec. Furthermore, the acoustic velocity of the bulk wave
propagating in a rotated Y cut LiTaO.sub.3 is constant regardless
of the rotation angle (Y cut). The acoustic velocity of the SV bulk
wave (slow transversal wave) is 3,367 m/sec, and the acoustic
velocity of the SH bulk wave (fast transversal wave) is 4,212
m/sec. Furthermore, in each of the first preferred embodiment and
the second comparative example, the aluminum nitride film is an
isotropic film, and the acoustic velocity of the bulk wave (S wave)
in the aluminum nitride film is 6,000 m/sec. Furthermore, the
silicon oxide film as the low-acoustic-velocity film 4 formed in
the first preferred embodiment is an isotropic film, and the
acoustic velocity of the bulk wave (S wave) in silicon oxide is
3,750 m/sec. Accordingly, since the dominant mode of the surface
acoustic wave propagating the piezoelectric is the U2 mode, the
following conditions are satisfied.
(1) Acoustic velocity of the bulk wave (S wave) in the
high-acoustic-velocity film: 6,000 m/sec>Acoustic velocity of
the dominant mode (U2) of the surface acoustic wave: 3,950 m/sec.
(2) Acoustic velocity of the bulk wave (S wave) in the
low-acoustic-velocity film: 3,750 m/sec<Acoustic velocity of the
bulk wave (SH) propagating in the piezoelectric film: 4,212
m/sec.
[0089] FIG. 2 shows the impedance-frequency characteristics of the
surface acoustic wave devices of the first preferred embodiment and
the first and second comparative examples, and FIG. 3 shows an
impedance Smith chart.
[0090] Furthermore, as shown in Table 3 below, in the surface
acoustic wave devices of the first preferred embodiment and the
first and second comparative examples, the Q factor at the resonant
frequency, the Q factor at the antiresonant frequency, the band
width ratio, and the TCF at the resonant frequency were obtained by
actual measurement.
[0091] The results are shown in Table 3 below.
TABLE-US-00003 TABLE 3 Q TCF Q (Anti- Band width [ppm/.degree. C.]
(Resonance) resonance) ratio [%] (Resonance) First 818 527 3.2 -45
comparative example Second 777 1285 4.1 -45 comparative example
First 1026 2080 4.4 -25 embodiment
[0092] In FIGS. 2 and 3, the solid line represents the results of
the first preferred embodiment, the dashed line represents the
results of the second comparative example, and the dotted-chain
line represents the results of the first comparative example.
[0093] As is clear from FIGS. 2 and 3, in the second comparative
example and the first preferred embodiment, the top-to-valley ratio
is higher than that in the first comparative example. The
top-to-valley ratio is a ratio of the impedance at an antiresonant
frequency to the impedance at a resonant frequency. As this value
increases, it becomes possible to configure a filter having a
higher Q factor and lower insertion loss. It is evident that, in
particular, in the first preferred embodiment, the top-to-valley
ratio is much higher than that in the second comparative example.
Furthermore, it is also evident that according to the first
preferred embodiment, the frequency difference between the resonant
frequency and the antiresonant frequency, i.e., the band width
ratio, can be increased compared with the second comparative
example.
[0094] Specifically, as is clear from Table 3, according to the
first preferred embodiment, the Q factor at the resonant frequency
can be increased, and in particular, the Q factor at the
antiresonant frequency can be greatly increased compared with the
first and second comparative examples. That is, since it is
possible to configure a one-port-type surface acoustic wave
resonator having a high Q factor, a filter having low insertion
loss can be configured using the surface acoustic wave device 1.
Furthermore, the band width ratio is 3.2% in the first comparative
example and 4.1% in the second comparative example. In contrast,
the band width ratio increases to 4.4% in the first preferred
embodiment.
[0095] In addition, as is clear from Table 3, according to the
first preferred embodiment, since the silicon oxide film is
disposed, the absolute value of TCF can be greatly decreased
compared with the first and second comparative examples.
[0096] FIGS. 5 and 6 show the results of FEM simulation, in which
the dotted-chain line represents the first preferred embodiment,
the dashed line represents the first comparative example, and the
solid line represents the second comparative example. In the FEM
simulation, a one-port resonator is assumed, in which duty=0.5, the
intersecting width is 20.lamda., and the number of pairs is
100.
[0097] As in the experimental results described above, in the FEM
simulation results, as is clear from FIG. 6, the Q factor can also
be increased compared with the first and second comparative
examples.
[0098] Consequently, as is clear from the experimental results and
the FEM simulation results regarding the first preferred embodiment
and the first and second comparative examples, it has been
confirmed that, by disposing the low-acoustic-velocity film 4
composed of silicon oxide between the high-acoustic-velocity film 3
composed of aluminum nitride and the piezoelectric film 5 composed
of LiTaO.sub.3, the Q factor can be enhanced. The reason for the
fact that the Q factor can be enhanced is believed to be that
energy of surface acoustic waves can be effectively confined to the
piezoelectric film 5, the low-acoustic-velocity film 4, and the
high-acoustic-velocity film 3 by the formation of the
high-acoustic-velocity film 3, and that the effect of suppressing
leakage of energy of surface acoustic waves outside the IDT
electrode can be enhanced by the formation of the
low-acoustic-velocity film 4.
[0099] Consequently, since the effect is obtained by disposing the
low-acoustic-velocity film 4 between the piezoelectric film 5 and
the high-acoustic-velocity film 3 as described above, the material
constituting the piezoelectric film is not limited to the
38.5.degree. Y cut LiTaO.sub.3 described above. The same effect can
be obtained in the case where LiTaO.sub.3 with other cut angles is
used. Furthermore, the same effect can be obtained in the case
where a piezoelectric single crystal such as LiNbO.sub.3 other than
LiTaO.sub.3, a piezoelectric thin film such as ZnO or AlN, or a
piezoelectric ceramic such as PZT is used.
[0100] Furthermore, the high-acoustic-velocity film 3 has a
function of confining the majority of energy of surface acoustic
waves to a portion in which the piezoelectric film and the
low-acoustic-velocity film 4 are stacked. Consequently, the
aluminum nitride film may be a c-axis-oriented, anisotropic film.
Furthermore, the material for the high-acoustic-velocity film 3 is
not limited to the aluminum nitride film, and it is expected that
the same effect can be obtained in the case where any of various
materials that can constitute the high-acoustic-velocity film 3
described above is used.
[0101] Furthermore, silicon oxide of the low-acoustic-velocity film
is not particularly limited as long as the acoustic velocity of a
bulk wave propagating therein is lower than the acoustic velocity
of a bulk wave propagating in the piezoelectric film. Consequently,
the material constituting the low-acoustic-velocity film 4 is not
limited to silicon oxide. Therefore, any of the various materials
described above as examples of a material that can constitute the
low-acoustic-velocity film 4 can be used.
Second Preferred Embodiment
[0102] Characteristics of a surface acoustic wave device according
to a second preferred embodiment having the structure described
below were simulated by a finite element method. The electrode
structure was the same as that shown in FIG. 1B.
[0103] An IDT electrode was an Al film with a thickness of
0.08.lamda.. A piezoelectric film was composed of 38.5.degree. Y
cut LiTaO.sub.3 film, and the thickness thereof was in a range of 0
to 3.lamda.. A low-acoustic-velocity film was composed of silicon
oxide, and the thickness thereof was 0 to 2.lamda.. A
high-acoustic-velocity film was composed of aluminum oxide, and the
thickness thereof was 1.5.lamda.. A supporting substrate was
composed of alumina.
[0104] The results are shown in FIGS. 7 to 10.
[0105] FIG. 7 is a graph showing the relationship between the
LiTaO.sub.3 film thickness, the acoustic velocity of the U2 mode
which is the usage mode, and the normalized film thickness of the
silicon oxide film. Furthermore, FIG. 8 is a graph showing the
relationship between the LiTaO.sub.3 film thickness, the
electromechanical coupling coefficient k.sup.2 of the U2 mode which
is the usage mode, and the normalized film thickness of the silicon
oxide film.
[0106] As is clear from FIG. 7, by forming the silicon oxide film,
the variations in acoustic velocity are small in the wide thickness
range of 0.05.lamda. to 0.5.lamda. of the piezoelectric film
composed of LiTaO.sub.3, in comparison with the case where the
normalized film thickness of the silicon oxide film is 0.0, i.e.,
no silicon oxide film is formed.
[0107] Furthermore, as is clear from FIG. 8, when the silicon oxide
film is formed, even in the case where the LiTaO.sub.3 film
thickness is small at 0.35.lamda. or less, by controlling the
silicon oxide film thickness, the electromechanical coupling
coefficient k.sup.2 can be increased to 0.08 or more, in comparison
with the case where no silicon oxide film is formed.
[0108] FIG. 9 is a graph showing the relationship between the
LiTaO.sub.3 film thickness, the temperature coefficient of
frequency TCV, and the normalized film thickness of the silicon
oxide film. FIG. 10 is a graph showing the relationship between the
LiTaO.sub.3 film thickness, the band width ratio, and the
normalized film thickness of the silicon oxide film.
[0109] Note that TCF=TCV-.alpha., where .alpha. is the coefficient
of linear expansion in the propagation direction. In the case of
LiTaO.sub.3, .alpha. is about 16 ppm/.degree. C.
[0110] As is clear from FIG. 9, by forming the silicon oxide film,
the absolute value of TCV can be further decreased in comparison
with the case where no silicon oxide film is formed. In addition,
as is clear from FIG. 10, even in the case where the thickness of
the piezoelectric film composed of LiTaO.sub.3 is small at about
0.35.lamda. or less, by adjusting the silicon oxide film thickness,
the band width ratio can be adjusted.
[0111] Furthermore, when the thickness of the silicon oxide film is
increased to more than about 2.lamda., stress is generated,
resulting in problems, such as warpage of the surface acoustic wave
device, which may cause handling difficulty. Consequently, the
thickness of the silicon oxide film is preferably about 2.lamda. or
less.
[0112] In the related art, it is known that, by using a laminated
structure in which an IDT is disposed on LiTaO.sub.3 and silicon
oxide is further disposed on the IDT, the absolute value of TCF in
the surface acoustic wave device can be decreased. However, as is
clear from FIG. 11, when the absolute value of TCV is intended to
be decreased, i.e., when the absolute value of TCF is intended to
be decreased, it is not possible to simultaneously achieve an
increase in the bandwidth ratio and a decrease in the absolute
value of the TCF. In contrast, by using the structure of the
present invention in which the high-acoustic-velocity film and the
low-acoustic-velocity film are stacked, a decrease in the absolute
value of TCF and an increase in the band width ratio can be
achieved. This will be described with reference to FIGS. 11 and
12.
[0113] FIG. 11 is a graph showing the relationship between the band
width ratio and the TCF in surface acoustic wave devices of third
to fifth comparative examples described below as conventional
surface acoustic wave devices.
[0114] Third comparative example: laminated structure of electrode
composed of Al/42.degree. Y cut LiTaO.sub.3. SH wave was used.
[0115] Fourth comparative example: laminated structure of silicon
oxide film/electrode composed of Cu/38.5.degree. Y cut LiTaO.sub.3
substrate. SH wave was used.
[0116] Fifth comparative example: laminated structure of silicon
oxide film/electrode composed of Cu/128.degree. Y cut LiNbO.sub.3
substrate. SV wave was used.
[0117] As is clear from FIG. 11, in any of the third comparative
example to the fifth comparative example, as the band width ratio
BW increases, the absolute value of TCF increases.
[0118] FIG. 12 is a graph showing the relationship between the band
width ratio BW (%) and the temperature coefficient of frequency TCV
in the case where the normalized film thickness of LiTaO.sub.3 was
changed in the range of about 0.1.lamda. to about 0.5.lamda. in
each of the thickness levels of the silicon oxide film of the
second preferred embodiment. As is clear from FIG. 12, in this
preferred embodiment, even in the case where the band width ratio
BW is increased, the absolute value of TCV does not increase. That
is, by adjusting the thickness of the silicon oxide film, the band
width ratio can be increased, and the absolute value of the
temperature coefficient of frequency TCV can be decreased.
[0119] That is, by stacking the low-acoustic-velocity film 4 and
the high-acoustic-velocity film 3 on the piezoelectric film
composed of LiTaO.sub.3, and in particular, by forming a silicon
oxide film as the low-acoustic-velocity film, it is possible to
provide an elastic wave device having a wide band width ratio and
good temperature characteristics.
[0120] Preferably, the coefficient of linear expansion of the
supporting substrate 2 is smaller than that of the piezoelectric
film 5. As a result, expansion due to heat generated in the
piezoelectric film 5 is restrained by the supporting substrate 2.
Consequently, the frequency temperature characteristics of the
elastic wave device can be further improved.
[0121] FIGS. 13 and 14 are graphs showing changes in the acoustic
velocity and changes in the band width ratio, respectively, with
changes in the thickness of the piezoelectric film composed of
LiTaO.sub.3 in the structure of the second preferred
embodiment.
[0122] As is clear from FIGS. 13 and 14, when the LiTaO.sub.3
thickness is about 1.5.lamda. or more, the acoustic velocity and
the band width ratio are nearly unchanged. The reason for this is
that energy of surface acoustic waves is confined to the
piezoelectric film and is not distributed into the
low-acoustic-velocity film 4 and the high-acoustic-velocity film 3.
Consequently, the effects of the low-acoustic-velocity film 4 and
the high-acoustic-velocity film 3 are not exhibited. Therefore, it
is more preferable to set the thickness of the piezoelectric film
to be about 1.5.lamda. or less. Thereby, it is believed that energy
of surface acoustic waves can be sufficiently distributed into the
low-acoustic-velocity film 4 and the Q factor can be further
enhanced.
[0123] The results of FIGS. 7 to 14 show that, by adjusting the
thickness of the silicon oxide film and the thickness of the
piezoelectric film composed of LiTaO.sub.3, the electromechanical
coupling coefficient can be adjusted over a wide range.
Furthermore, it is clear that when the thickness of the
piezoelectric film composed of LiTaO.sub.3 is in the range of about
0.05.lamda. to about 0.5.lamda., the electromechanical coupling
coefficient can be adjusted in a wider range. Consequently, the
thickness of the piezoelectric film composed of LiTaO.sub.3 is
preferably in the range of about 0.05.lamda. to about
0.5.lamda..
[0124] Conventionally, it has been required to adjust cut angles of
the piezoelectric used in order to adjust the electromechanical
coupling coefficient. However, when the cut angles, i.e., Euler
angles, are changed, other material characteristics, such as the
acoustic velocity, temperature characteristics, and spurious
characteristics, are also changed. Consequently, it has been
difficult to simultaneously satisfy these characteristics, and
optimization of design has been difficult.
[0125] However, as is clear from the results of the second
preferred embodiment described above, according to the present
invention, even in the case where a piezoelectric single crystal
with the same cut angles is used as the piezoelectric film, by
adjusting the thickness of the silicon oxide film, i.e., the
low-acoustic-velocity film, and the thickness of the piezoelectric
film, the electromechanical coupling coefficient can be freely
adjusted. Consequently, freedom of design can be greatly increased.
Therefore, it is enabled to simultaneously satisfy various
characteristics, such as the acoustic velocity, the
electromechanical coupling coefficient, frequency temperature
characteristics, and spurious characteristics, and it is possible
to easily provide a surface acoustic wave device having desired
characteristics.
Third Preferred Embodiment
[0126] As a third preferred embodiment, surface acoustic wave
devices same as those of the first preferred embodiment were
fabricated. The materials and thickness were as described
below.
[0127] A laminated structure included an Al film with a thickness
of 0.08.lamda. as an IDT electrode 6/a LiTaO.sub.3 film with a
thickness of 0.25.lamda. as a piezoelectric film 4/a silicon oxide
film with a thickness in the range of 0 to 2.lamda. as a
low-acoustic-velocity film 4/a high-acoustic-velocity film. As the
high-acoustic-velocity film, a silicon nitride film, an aluminum
oxide film, or diamond was used. The thickness of the
high-acoustic-velocity film 3 was 1.5.lamda..
[0128] FIGS. 15 and 16 are graphs showing the relationship between
the thickness of the silicon oxide film and the acoustic velocity
and the relationship between the thickness of the silicon oxide
film and the electromechanical coupling coefficient k.sup.2,
respectively, in the third preferred embodiment.
[0129] The acoustic velocity of the bulk wave (S wave) in the
silicon nitride film is 6,000 m/sec, and the acoustic velocity of
the bulk wave (S wave) in aluminum oxide is 6,000 m/sec.
Furthermore, the acoustic velocity of the bulk wave (S wave) in
diamond is 12,800 m/sec.
[0130] As is clear from FIGS. 15 and 16, as long as the
high-acoustic-velocity film 4 satisfies the conditions for the
high-acoustic-velocity film 4 described earlier, even if the
material for the high-acoustic-velocity film 4 and the thickness of
the silicon oxide film are changed, the electromechanical coupling
coefficient and the acoustic velocity are nearly unchanged. In
particular, if the thickness of the silicon oxide film is about
0.1.lamda. or more, the electromechanical coupling coefficient is
nearly unchanged in the silicon oxide film thickness range of about
0.1.lamda. to about 0.5.lamda. regardless of the material for the
high-acoustic-velocity film. Furthermore, as is clear from FIG. 15,
in the silicon oxide film thickness range of about 0.3.lamda. to
about 2.lamda., the acoustic velocity is nearly unchanged
regardless of the material for the high-acoustic-velocity film.
[0131] Consequently, in the present invention, the material for the
high-acoustic-velocity film is not particularly limited as long as
the above conditions are satisfied.
Fourth Preferred Embodiment
[0132] In a fourth preferred embodiment, while changing the Euler
angles (0.degree., .theta., .psi.) of the piezoelectric film, the
electromechanical coupling coefficient of a surface acoustic wave
containing as a major component the U2 component (SH component) was
measured.
[0133] A laminated structure was composed of IDT electrode
6/piezoelectric film 5/low-acoustic-velocity film
4/high-acoustic-velocity film 3/supporting substrate 2. As the IDT
electrode 6, Al with a thickness of 0.08.lamda. was used. As the
piezoelectric film, LiTaO.sub.3 with a thickness of 0.25.lamda. was
used. As the low-acoustic-velocity film 4, silicon oxide with a
thickness of 0.35.lamda. was used. As the high-acoustic-velocity
film 3, an aluminum nitride film with a thickness of 1.5.lamda. was
used. As the supporting substrate 2, glass was used.
[0134] In the structure described above, regarding many surface
acoustic wave devices with Euler angles (0.degree., .theta., .psi.)
in which .theta. and .psi. were varied, the electromechanical
coupling coefficient was obtained by FEM. As a result, it was
confirmed that in a plurality of regions R1 shown in FIG. 17, the
electromechanical coupling coefficient k.sup.2 of the mode mainly
composed of the U2 component (SH component) is about 2% or more.
Note that the same results were obtained in the range of Euler
angles (0.degree..+-.5, .theta., .psi.).
[0135] That is, when LiTaO.sub.3 with Euler angles located in a
plurality of ranges R1 shown in FIG. 17 is used, the
electromechanical coupling coefficient of the vibration mainly
composed of the U2 component is about 2% or more. Therefore, it is
clear that a bandpass filter with a wide band width can be
configured using a surface acoustic wave device according to a
preferred embodiment of the present invention.
Fifth Preferred Embodiment
[0136] Assuming the same structure as that in the fourth preferred
embodiment, the electromechanical coupling coefficient of a surface
acoustic wave mainly composed of the U3 component (SV component)
was obtained by FEM. The range of Euler angles in which the
electromechanical coupling coefficient of the mode mainly composed
of the U2 (SH component) is about 2% or more, and the
electromechanical coupling coefficient of the mode mainly composed
of the U3 (SV component) is about 1% or less was obtained. The
results are shown in FIG. 18. In a plurality of ranges R2 shown in
FIG. 18, the electromechanical coupling coefficient of the mode
mainly composed of the U2 (SH component) is about 2% or more, and
the electromechanical coupling coefficient of the mode mainly
composed of the U3 (SV component) is about 1% or less.
Consequently, by using LiTaO.sub.3 with Euler angles located in any
one of a plurality of regions R2, the electromechanical coupling
coefficient of the U2 mode used can be increased and the
electromechanical coupling coefficient of the U3 mode which is
spurious can be decreased. Therefore, it is possible to configure a
bandpass filter having better filter characteristics.
Sixth Preferred Embodiment
[0137] As in the second preferred embodiment, simulation was
carried out on a surface acoustic wave device having the structure
described below. As shown in Table 4 below, in the case where the
transversal wave acoustic velocity of the low-acoustic-velocity
film and the specific acoustic impedance of the transversal wave of
the low-acoustic-velocity film were changed in 10 levels,
characteristics of surface acoustic waves mainly composed of the U2
component were simulated by a finite element method. In the
transversal wave acoustic velocity and specific acoustic impedance
of the low-acoustic-velocity film, the density and elastic constant
of the low-acoustic-velocity film were changed. Furthermore, as the
material constants of the low-acoustic-velocity film not shown in
Table 4, material constants of silicon oxide were used.
TABLE-US-00004 TABLE 4 Specific Specific acoustic gravity Elastic
constant Transversal wave impedance of .rho. C11 C44 acoustic
velocity transversal wave Level [kg/m.sup.3] [N/m.sup.2]
[N/m.sup.2] V [m/s] Zs [N s/m.sup.3] Remarks 1 1.11E+03 4.73E+10
1.56E+10 3757 4.2.E+06 2 2.21E+03 7.85E+10 3.12E+10 3757 8.3.E+06
Silicon oxide equivalent 3 3.32E+03 1.10E+11 4.68E+10 3757 1.2.E+07
4 6.63E+03 2.03E+11 9.36E+10 3757 2.5.E+07 5 1.11E+04 3.28E+11
1.56E+11 3757 4.2.E+07 6 2.21E+03 3.17E+10 7.80E+09 1879 4.2.E+06 7
4.42E+03 4.73E+10 1.56E+10 1879 8.3.E+06 8 6.63E+03 6.29E+10
2.34E+10 1879 1.2.E+07 9 1.33E+04 1.10E+11 4.68E+10 1879 2.5.E+07
10 2.21E+04 1.72E+11 7.80E+10 1879 4.2.E+07 Note that, in Table 4,
1.11E+03 means 1.11 .times. 10.sup.3. That is, aE + b represents a
.times. 10.sup.b.
[0138] The electrode structure was the same as that shown in FIG.
1B, and the surface acoustic wave device had a laminated structure
of IDT electrode/piezoelectric film/low-acoustic-velocity
film/high-acoustic-velocity film/supporting substrate. The IDT
electrode was an Al film with a thickness of 0.08.lamda.. The
piezoelectric film was composed of 40.degree. Y cut LiTaO.sub.3. In
each of the cases where the thickness of the piezoelectric film was
0.1.lamda., 0.4.lamda., and 0.6.lamda., 10 levels shown in Table 4
were calculated. The thickness of the low-acoustic-velocity film
was 0.4.lamda.. The high-acoustic-velocity film was composed of
aluminum oxide, and the thickness thereof was 1.5.lamda.. The
supporting substrate was composed of an alumina substrate.
[0139] FIGS. 19A to 19C are graphs showing the relationships
between the specific acoustic impedance of the
low-acoustic-velocity film and the band width ratio in the sixth
preferred embodiment. In the graphs, each level shows the behavior
in the case where the acoustic velocity of the transversal wave in
the low-acoustic-velocity film changes, and the band width ratio in
each level is normalized to the band width ratio in the case where
the specific acoustic impedance of the piezoelectric film is equal
to the specific acoustic impedance of the low-acoustic-velocity
film. The specific acoustic impedance is expressed as a product of
the acoustic velocity of the bulk wave and the density of the
medium. In the sixth preferred embodiment, the bulk wave of the
piezoelectric film is the SH bulk wave, the acoustic velocity is
4,212 m/s, and the density is 7.454.times.10.sup.3 kg/m.sup.3.
[0140] Consequently, the specific acoustic impedance of the
piezoelectric film is 3.14.times.10.sup.7 Ns/m.sup.3. Furthermore,
regarding the acoustic velocity of the bulk wave used for
calculating the specific acoustic impedance of each of the
low-acoustic-velocity film and the piezoelectric film, for the
dominant mode of the elastic wave shown in the left column of Table
1 or 2, the acoustic velocity is determined according to the mode
of the bulk wave shown in the right column of Table 1 or 2.
[0141] Furthermore, FIGS. 20A to 20C are graphs showing the
relationships between the specific acoustic impedance of the
transversal wave of the low-acoustic-velocity film and the acoustic
velocity of the propagating surface acoustic wave in the sixth
preferred embodiment.
[0142] As is clear from FIGS. 19A to 19C, regardless of the
thickness of the piezoelectric film, the band width ratio increases
as the specific acoustic impedance of the low-acoustic-velocity
film becomes smaller than the specific acoustic impedance of the
piezoelectric film. The reason for this is that since the specific
acoustic impedance of the low-acoustic-velocity film is smaller
than the specific acoustic impedance of the piezoelectric film, the
displacement of the piezoelectric film under certain stress
increases, thus generating a larger electric charge, and therefore,
equivalently higher piezoelectricity can be obtained. That is,
since this effect is obtained depending only on the magnitude of
specific acoustic impedance, regardless of the vibration mode of
the surface acoustic wave, the type of the piezoelectric film, or
the type of the low-acoustic-velocity film, it is possible to
obtain a surface acoustic wave device having a higher band width
ratio when the specific acoustic impedance of the
low-acoustic-velocity film is smaller than the specific impedance
of the piezoelectric film.
[0143] In each of the first to sixth preferred embodiments of the
present invention, the IDT electrode 6, the piezoelectric film 5,
the low-acoustic-velocity film 4, the high-acoustic-velocity film
3, and the supporting substrate 2 preferably are stacked in that
order from the top, for example. However, within the extent that
does not greatly affect the propagating surface acoustic wave and
boundary wave, an adhesion layer composed of Ti, NiCr, or the like,
an underlying film, or any medium may be disposed between the
individual layers. In such a case, the same effect can be obtained.
For example, a new high-acoustic-velocity film which is
sufficiently thin compared with the wavelength of the surface
acoustic wave may be disposed between the piezoelectric film 5 and
the low-acoustic-velocity film 4. In such a case, the same effect
can be obtained. Furthermore, energy of the mainly used surface
acoustic wave is not distributed between the high-acoustic-velocity
film 3 and the supporting substrate 2. Consequently, any medium
with any thickness may be disposed between the
high-acoustic-velocity film 3 and the supporting substrate 2. In
such a case, the same advantageous effects can be obtained.
[0144] The seventh and eighth preferred embodiments described below
relate to surface acoustic wave devices provided with such a medium
layer.
Seventh Preferred Embodiment
[0145] In a surface acoustic wave device 21 according to a seventh
preferred embodiment shown in FIG. 23, a medium layer 22 is
disposed between a supporting substrate 2 and a
high-acoustic-velocity film 3. The structure other than this is the
same as that in the first preferred embodiment. Therefore, the
description of the first preferred embodiment is incorporated
herein. In the surface acoustic wave device 21, an IDT electrode 6,
a piezoelectric film 5, a low-acoustic-velocity film 4, the
high-acoustic-velocity film 3, the medium layer 22, and the
supporting substrate 2 are stacked in that order from the top.
[0146] As the medium layer 22, any material, such as a dielectric,
a piezoelectric, a semiconductor, or a metal, may be used. Even in
such a case, the same effect as that of the first preferred
embodiment can be obtained. In the case where the medium layer 22
is composed of a metal, the band width ratio can be decreased.
Consequently, in the application in which the band width ratio is
small, the medium layer 22 is preferably composed of a metal.
Eighth Preferred Embodiment
[0147] In a surface acoustic wave device 23 according to an eighth
preferred embodiment shown in FIG. 24, a medium layer 22 and a
medium layer 24 are disposed between a supporting substrate 2 and a
high-acoustic-velocity film 3. That is, an IDT electrode 6, a
piezoelectric film 5, a low-acoustic-velocity film 4, the
high-acoustic-velocity film 3, the medium layer 22, the medium
layer 24, and the supporting substrate 2 are stacked in that order
from the top. Other than the medium layer 22 and the medium layer
24, the structure is the same as that in the first preferred
embodiment.
[0148] The medium layers 22 and 24 may be composed of any material,
such as a dielectric, a piezoelectric, a semiconductor, or a metal.
Even in such a case, in the eighth preferred embodiment, it is
possible to obtain the same effect as that of the surface acoustic
wave device of the first preferred embodiment.
[0149] In this preferred embodiment, after a laminated structure
including the piezoelectric film 5, the low-acoustic-velocity film
4, the high-acoustic-velocity film 3, and the medium layer 22 and a
laminated structure including the medium layer 24 and the
supporting substrate 2 are separately fabricated, both laminated
structures are bonded to each other. Then, the IDT electrode 6 is
formed on the piezoelectric film 5. As a result, it is possible to
obtain a surface acoustic wave device according to this preferred
embodiment without being restricted by manufacturing conditions
when each laminated structure is fabricated. Consequently, freedom
of selection for materials constituting the individual layers can
be increased.
[0150] When the two laminated structures are bonded to each other,
any joining method can be used. For such a bonded structure,
various methods, such as bonding by hydrophilization, activation
bonding, atomic diffusion bonding, metal diffusion bonding, anodic
bonding, bonding using a resin or SOG, can be used. Furthermore,
the joint interface between the two laminated structures is located
on the side opposite to the piezoelectric film 5 side of the
high-acoustic-velocity film 3. Consequently, the joint interface
exists in the portion below the high-acoustic-velocity film 3 in
which major energy of the surface acoustic wave used is not
distributed. Therefore, surface acoustic wave propagation
characteristics are not affected by the quality of the joint
interface. Accordingly, it is possible to obtain stable and good
resonance characteristics and filter characteristics.
Ninth Preferred Embodiment
[0151] In a surface acoustic wave device 31 shown in FIG. 25, an
IDT electrode 6, a piezoelectric film 5, a low-acoustic-velocity
film 4, and a high-acoustic-velocity supporting substrate 33 which
also functions as a high-acoustic-velocity film are stacked in that
order from the top. That is, the high-acoustic-velocity supporting
substrate 33 serves both as the high-acoustic-velocity film 3 and
as the supporting substrate 2 in the first preferred embodiment.
Consequently, the acoustic velocity of a bulk wave in the
high-acoustic-velocity supporting substrate 33 is set to be higher
than the acoustic velocity of a surface acoustic wave propagating
in the piezoelectric film 5. Thus, the same effect as that in the
first preferred embodiment can be obtained. Moreover, since the
high-acoustic-velocity supporting substrate 33 serves both as the
high-acoustic-velocity film and as the supporting substrate, the
number of components can be reduced.
Tenth Preferred Embodiment
[0152] In a tenth preferred embodiment, the relationship between
the Q factor and the frequency in a one-port-type surface acoustic
wave resonator as a surface acoustic wave device was simulated by
FEM.
[0153] Here, as the surface acoustic wave device according to the
first preferred embodiment, shown in FIGS. 1A and 1B, the following
structure was assumed.
[0154] The structure included an IDT electrode 6 composed of Al
with a thickness of 0.1.lamda., a piezoelectric film composed of a
50.degree. Y cut LiTaO.sub.3 film, an SiO.sub.2 film as a
low-acoustic-velocity film, an aluminum nitride film with a
thickness of 1.5.lamda. as a high-acoustic-velocity film, an
SiO.sub.2 film with a thickness of 0.3.lamda., and a supporting
substrate composed of alumina stacked in that order from the top.
In this simulation, the thickness of the LiTaO.sub.3 film as the
piezoelectric film was changed to 0.15.lamda., 0.20.lamda.,
0.25.lamda., or 0.30.lamda.. Furthermore, the thickness of the
SiO.sub.2 film as the low-acoustic-velocity film was changed in the
range of 0 to 2.lamda..
[0155] The duty of the IDT electrode was 0.5, the intersecting
width of electrode fingers was 20.lamda., and the number of
electrode finger pairs was 100.
[0156] For comparison, a one-port-type surface acoustic wave
resonator, in which an IDT electrode composed of Al with a
thickness of 0.1.lamda. and a 38.5.degree. Y cut LiTaO.sub.3
substrate were stacked in that order from the top, was prepared.
That is, in the comparative example, an electrode structure
including the IDT electrode composed of Al is disposed on a
38.5.degree. Y cut LiTaO.sub.3 substrate with a thickness of 350
.mu.m.
[0157] Regarding the surface acoustic wave devices according to the
tenth preferred embodiment and the comparative example, the
relationship between the Q factor and the frequency was obtained by
simulation by FEM. In the range from the resonant frequency at
which the impedance of the one-port resonator was lowest to the
antiresonant frequency at which the impedance was highest, the
highest Q factor was defined as the Q.sub.max factor. A higher
Q.sub.max factor indicates lower loss.
[0158] The Q.sub.max factor of the comparative example was 857.
FIG. 26 shows the relationship between the LiTaO.sub.3 film
thickness, the SiO.sub.2 film thickness, and the Q.sub.max in this
preferred embodiment.
[0159] As is clear from FIG. 26, in each case where the LiTaO.sub.3
film thickness is 0.15.lamda., 0.20.lamda., 0.25.lamda., or
0.30.lamda., the Q.sub.max factor increases when the thickness of
the low-acoustic-velocity film composed of SiO.sub.2 exceeds 0. It
is also clear that in the tenth preferred embodiment, in any of the
cases, the Q.sub.max factor is effectively enhanced relative to the
comparative example.
Preferred Embodiment of Manufacturing Method
[0160] The elastic wave device according to the first preferred
embodiment includes, as described above, the high-acoustic-velocity
film 3, the low-acoustic-velocity film 4, the piezoelectric film 5,
and the IDT electrode 6 which are disposed on the supporting
substrate 2. The method for manufacturing such an elastic wave
device is not particularly limited. By using a manufacturing method
using the ion implantation process described below, it is possible
to easily obtain an elastic wave device 1 having a piezoelectric
film with a small thickness. A preferred embodiment of the
manufacturing method will be described with reference to FIGS.
21A-21E and 22A-22C.
[0161] First, as shown in FIG. 21A, a piezoelectric substrate 5A is
prepared. In this preferred embodiment, the piezoelectric substrate
5A is preferably composed of LiTaO.sub.3. Hydrogen ions are
implanted from a surface of the piezoelectric substrate 5A. The
ions to be implanted are not limited to hydrogen, and helium or the
like may be used.
[0162] In the ion implantation, energy is not particularly limited.
In this preferred embodiment, preferably the energy is about 107
KeV, and the dose amount is about 8.times.10.sup.16 atoms/cm.sup.2,
for example.
[0163] When ion implantation is performed, the ion concentration is
distributed in the thickness direction in the piezoelectric
substrate 5A. In FIG. 21A, the dashed line represents a region in
which the ion concentration is highest. In a high concentration
ion-implanted region 5a in which the ion concentration is highest
represented by the dashed line, when heating is performed as will
be described later, separation easily occurs owing to stress. Such
a method in which separation is performed using the high
concentration ion-implanted region 5a is disclosed in Japanese
Unexamined Patent Application Publication No. 2002-534886.
[0164] In this step, at the high concentration ion-implanted region
5a, the piezoelectric substrate 5A is separated to obtain a
piezoelectric film 5. The piezoelectric film 5 is a layer between
the high concentration ion-implanted region 5a and the surface of
the piezoelectric substrate from which ion implantation is
performed. In some cases, the piezoelectric film 5 may be subjected
to machining, such as grinding. Consequently, the distance from the
high concentration ion-implanted region 5a to the surface of the
piezoelectric substrate on the ion implantation side is set to be
equal to or slightly larger than the thickness of the finally
formed piezoelectric film.
[0165] Next, as shown in FIG. 21B, a low-acoustic-velocity film 4
is formed on the surface of the piezoelectric substrate 5A on which
the ion implantation has been performed. In addition, a
low-acoustic-velocity film formed in advance may be bonded to the
piezoelectric substrate 5A using a transfer method or the like.
[0166] Next, as shown in FIG. 21C, a high-acoustic-velocity film 3
is formed on a surface of the low-acoustic-velocity film 4,
opposite to the piezoelectric substrate 5A side of the
low-acoustic-velocity film 4. Instead of using the film formation
method, the high-acoustic-velocity film 3 may also be bonded to the
low-acoustic-velocity film 4 using a transfer method or the
like.
[0167] Furthermore, as shown in FIG. 21D, an exposed surface of the
high-acoustic-velocity film 3, opposite to the
low-acoustic-velocity film 4 side of the high-acoustic-velocity
film 3, is subjected to mirror finishing. By performing mirror
finishing, it is possible to strengthen bonding between the
high-acoustic-velocity film and the supporting substrate which will
be described later.
[0168] Then, as shown in FIG. 21E, a supporting substrate 2 is
bonded to the high-acoustic-velocity film 3.
[0169] As the low-acoustic-velocity film 4, as in the first
preferred embodiment, a silicon oxide film is used. As the
high-acoustic-velocity film 3, an aluminum nitride film is
used.
[0170] Next, as shown in FIG. 22A, after heating, a piezoelectric
substrate portion 5b located above the high concentration
ion-implanted region 5a in the piezoelectric substrate 5A is
separated. As described above, by applying stress by heating
through the high concentration ion-implanted region 5a, the
piezoelectric substrate 5A becomes easily separated. In this case,
the heating temperature may be set at about 250.degree. C. to
400.degree. C., for example.
[0171] In this preferred embodiment, by the heat-separation, a
piezoelectric film 5 with a thickness of about 500 nm, for example,
is obtained. In such a manner, as shown in FIG. 22B, a structure in
which the piezoelectric film 5 is stacked on the
low-acoustic-velocity film 4 is obtained. Then, in order to recover
piezoelectricity, heat treatment is performed in which the
structure is retained at a temperature of about 400.degree. C. to
about 500.degree. C. for about 3 hours, for example. Optionally,
prior to the heat treatment, the upper surface of the piezoelectric
film 5 after the separation may be subjected to grinding.
[0172] Then, as shown in FIG. 22C, an electrode including an IDT
electrode 6 is formed. The electrode formation method is not
particularly limited, and an appropriate method, such as vapor
deposition, plating, or sputtering, may be used, for example.
[0173] According to the manufacturing method of this preferred
embodiment, by the separation, it is possible to easily form a
piezoelectric film 5 with rotated Euler angles at a uniform
thickness.
Eleventh Preferred Embodiment
[0174] In the first preferred embodiment, the IDT electrode 6, the
piezoelectric film 5, the low-acoustic-velocity film 4, the
high-acoustic-velocity film 3, and the supporting substrate 2 are
preferably stacked in that order from the top. In a surface
acoustic wave device 41 according to an eleventh preferred
embodiment shown in FIG. 27, a dielectric film 42 may be arranged
so as to cover an IDT electrode 6. By disposing such a dielectric
film 42, frequency temperature characteristics can be adjusted, and
moisture resistance can be enhanced.
Twelfth Preferred Embodiment
[0175] In the preferred embodiments described above, description
has been provided for surface acoustic wave devices. The present
invention can also be applied to other elastic wave devices, such
as boundary acoustic wave devices. In such a case, the same
advantageous effects can also be obtained. FIG. 28 is a schematic
elevational cross-sectional view of a boundary acoustic wave device
43 according to a twelfth preferred embodiment. In this case, a
low-acoustic-velocity film 4, a high-acoustic-velocity film 3, and
a supporting substrate 2 are preferably stacked in that order from
the top, under a piezoelectric film 5. This structure is preferably
the same or substantially the same as that of the first preferred
embodiment. In order to excite a boundary acoustic wave, an IDT
electrode 6 is provided at the interface between the piezoelectric
film 5 and a dielectric 44 disposed on the piezoelectric film
5.
[0176] Furthermore, FIG. 29 is a schematic elevational
cross-sectional view of a boundary acoustic wave device 45 having a
three-medium structure. In this case, with respect to a structure
in which a low-acoustic-velocity film 4, a high-acoustic-velocity
film 3, and a supporting substrate 2 are stacked in that order,
under a piezoelectric film 5, an IDT electrode 6 is provided at the
interface between the piezoelectric film 5 and a dielectric film
46. Furthermore, a dielectric 47 in which the acoustic velocity of
a transversal wave propagating therein is faster than that of the
dielectric 46 is disposed on the dielectric 46. As a result, a
boundary acoustic wave having a three-medium structure is
provided.
[0177] In the boundary acoustic wave device, such as the boundary
acoustic wave device 43 or 45, as in the surface acoustic wave
device 1 according to the first preferred embodiment, by disposing
a laminated structure composed of low-acoustic-velocity film
4/high-acoustic-velocity film 3 on the lower side of the
piezoelectric film 5, the same effect as that in the first
preferred embodiment can be obtained.
[0178] While preferred embodiments of the present invention have
been described above, it is to be understood that variations and
modifications will be apparent to those skilled in the art without
departing from the scope and spirit of the present invention. The
scope of the present invention, therefore, is to be determined
solely by the following claims.
* * * * *